Silicon nitride powders are contaminated above all by a certain amount of oxygen. In commercial powders, the oxygen content is between about 0.8% by weight and 2.5% by weight (G. Ziegler, J. Heinrich, G. Wotting, J. Mater. Sci. 22 (1987), 3041-3086).
Although the oxygen is really regarded as contaminating the powder, it is actually desired to a limited extent for sintering processes carried out in the absence of pressure or under gas pressure to improve the sintering activity of the silicon nitride power. Sintering additives are added to the powder for sintering, forming liquid phases together with the oxygen present in the powder at the sintering temperature and thus enabling the material to be compacted. Overall, an oxygen content of approximately 1.5% by weight is regarded as optimal for sintering under gas pressure.
However, the oxygen has an adverse effect on the mechanical properties, particularly at high temperatures, because it lower the glass temperature of the secondary phases and hence reduces strength and creep resistance at high temperatures.
For hot isostatic sintering, however, the content of sintering additives and hence the oxygen content can be lower. Moldings produced by hot isostatic pressing from Si.sub.3 N.sub.4 powders having an oxygen content described as particularly low of 0.8% by weight show particularly good mechanical properties (R. R. Wills, M. C. Brockway), L. G. McCoy, D. E. Niesz, Ceram. Eng. Sci. Proc. 1 (1980), pages 34-39). However, even under inert or reducing conditions as described in U.S. Pat. No. 4,405,589, it has not hitherto been possible to obtain powders having a lower oxygen content than 0.8% by weight.
Accordingly, the object of the present invention is to provide Si.sub.3 N.sub.4 powders which do not have any of the described disadvantages of the prior art.